This invention relates in general to the field of telecommunications and fiber optics and more particularly to a composite optical fiber transmission line and method.
Advances in fiber optic technology and fiber optic transmission systems are revolutionizing telecommunications. The main driving force behind this revolution is the promise of extremely high communications bandwidth. A single beam of modulated laser light can carry vast amounts of information that is equal to literally hundreds of thousands of phone calls or hundreds of video channels. Over the past few years, this technology has advanced at such a pace that the bandwidth capabilities have more than doubled every two years. The bandwidth strides have come about through major milestones, breakthroughs, and improvements in various areas such as fiber optic materials and transmitter devices. As a result, bandwidth capability or data rates, which may be expressed in terms of digital bits per second (xe2x80x9cbpsxe2x80x9d), have escalated. In some cases, for example, capacity has increased from 500 Mbps to 10 Gbps and higher.
In a fiber optic transmission system, a digital signal is represented by an optical signal. The optical signal is generated by modulating a laser light or rapidly turning a laser light on and off to represent the various xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d or xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d values or states of a digital signal. This may be referred to as amplitude modulation. The laser light, or optical signal, is generally emitted from a laser of an optical transmitter. In the frequency domain, this signal includes numerous frequency components spaced very closely about the nominal center frequency of the optical carrier, such as, for example, 193,000 Ghz.
To increase the overall data rate or bandwidth of a fiber optic transmission system, multiple optical signals may be multiplexed using Wavelength Division Multiplexing (xe2x80x9cWDMxe2x80x9d) or Dense Wideband Division Multiplexing (xe2x80x9cDWDMxe2x80x9d). WDM and DWDM both involve simultaneously transmitting two or more optical signals, each at a different wavelength or frequency, through an optical fiber in the same direction.
WDM has been used to refer to multiplexing or combining two or four optical signals, while DWDM has been used to refer to multiplexing or combining eight, sixteen, and even forty or more optical signals. Each wavelength of a WDM or DWDM optical signal is a virtual optical channel that may support, for example, data rates of OC-48 at 2.5 Gbps or OC-192 at 9.953 Gbps to provide a significant overall data rate. Optical Erbium-Doped Fiber Amplifiers (xe2x80x9cEDFAxe2x80x9d) are typically used at repeaters to simultaneously and directly boost all wavelengths or virtual optical channels of such WDM or DWDM optical signals. This provides the tremendous advantage of eliminating the requirement of separating each WDM or DWDM optical signal into its various optical signals of different wavelengths, converting each such optical signal to its electrical signal equivalent, amplifying each such electrical signal, and then combining or multiplexing the various signals to regenerate the WDM or DWDM optical signal.
Unfortunately, WDM and DWDM may create wave mixing, such as four-wave mixing, between the various optical signal wavelengths of the WDM or DWDM optical signal. This can increase the Bit Error Rate (xe2x80x9cBERxe2x80x9d) of the optical fiber transmission system. Further, WDM and DWDM optical signals are provided at higher power levels and require optical fibers capable of handling the higher power levels. While it is difficult to design an optical fiber transmission system to accommodate an optical signal at a single wavelength, it is exceedingly more challenging and difficult to design an optical fiber transmission line that can simultaneously accommodate multiple wavelength optical signals, such as WDM and DWDM optical signals.
An optical signal is transmitted in a fiber optic transmission system using, generally, an optical transmitter, which includes a light source or laser, an optical fiber, an optical amplifier, and an optical receiver. A modulated optical signal arriving at an optical receiver must be of sufficient quality to allow the receiver to clearly distinguish the on-and-off pattern of light pulses sent by the transmitter. Noise, attenuation, modal dispersion, chromatic dispersion, chromatic dispersion slope, polarization mode dispersion, and wave mixing are some of the impairments that can distort an optical signal and render the optical signal marginal or unusable at the receiver. The distortion of an optical signal makes it extremely difficult or impossible for an optical receiver to accurately detect or reconstitute the digital signal. This is because distortion nonuniformly broadens, spreads, or widens the various light pulses resulting in such closely spaced pulses or overlapping pulses that the pulses are virtually indistinguishable from one another.
Conventionally, a properly designed fiber optic transmission system or channel can maintain a BER of 10xe2x88x9213 or better. When an optical channel degrades to a BER of 10xe2x88x928, a telecommunications system automatically switches to an alternate optical channel in an attempt to improve the BER. Otherwise, the optical channel operates at a reduced or lower data rate or bandwidth, which harms overall system performance. All types of dispersion, modal, chromatic, and polarization mode, make the BER worse.
The negative effects of modal dispersion have been dramatically reduced and, in some cases, effectively eliminated through the use of single-mode fiber. Single-mode fiber prevents or reduces the ability of an optical signal to take multiple or different paths through an optical fiber. This prevents or reduces multimode distortion. Single-mode fiber allows only a single mode of light to propagate through the fiber. Single-mode fibers generally achieve this through the use of a smaller core, as compared to multimode fibers.
Chromatic dispersion and Polarization Mode Dispersion (xe2x80x9cPMDxe2x80x9d) remain major contributors to distortion of an optical signal, which increases the BER of the optical channel. The distortion caused by chromatic dispersion and polarization mode dispersion generally increases as the bandwidth or data rate increases and as the optical fiber transmission distance increases.
Chromatic dispersion and PMD have been identified as the major contributors to distortion. Chromatic dispersion has received the far greater attention because its adverse effects were initially more limiting at the then available bandwidth and data rate of the leading edge in optical fiber transmission systems. More recently, it has been recognized that PMD is one of the limiting factors that must be overcome to take telecommunications and fiber optic transmission systems to the next level and to continue with the heretofore rapid increase and expansion of bandwidth and data rates. Developments have been made and continue to be made to address problems and limitations caused by PMD.
Chromatic dispersion occurs when the various frequency components or colors that make up a pulse of laser light travel at different speeds through an optical fiber and arrive at the optical receiver at different times. This occurs because the index of refraction of a material, such as an optical fiber, varies with frequency or wavelength. As a result, the various pulses of light that make up an optical signal are distorted through pulse spreading, making it difficult or impossible to accurately receive and recover the digital data contained in the optical signal. Chromatic dispersion presents problems when it is too high and when it is too low. In addition to a distorted optical signal and a reduced data rate, high chromatic dispersion may also result in self phase modulation and generally requires the use of a long dispersion compensating fiber. When chromatic dispersion is too low, the problem of cross-phase modulation may present significant limitations. Thus, it may not be desirable to completely eliminate chromatic dispersion in all cases. Chromatic dispersion may be thought of as the amount of scattering that occurs in the optical signal as it travels through a fiber optic transmission path or medium.
The following single-mode fibers have provided improvements in addressing the problem of chromatic dispersion: (1) Standard Single-Mode Fiber (xe2x80x9cSSMFxe2x80x9d), (2) Dispersion Shifted Fiber (xe2x80x9cDSFxe2x80x9d), and (3) modified or Nonzero Dispersion Shifted Fiber (xe2x80x9cNZDSFxe2x80x9d). The use of Distributed Feedback (xe2x80x9cDFBxe2x80x9d) lasers, which provide narrow output spectra, with these single-mode fibers have resulted in significant improvements and increased bandwidth. DFB lasers provide a light source to use with single-mode optical fibers. DFB lasers produce a light with an extremely narrow distribution of output frequencies and wavelengths. This minimizes the chromatic dispersion problem caused by the fact that different wavelengths travel at slightly different speeds through a fiber.
SSMF, which does not have its chromatic dispersion characteristics shifted, is a single-mode fiber that is implemented in an optical fiber transmission system to account for or reduce the effects caused by chromatic dispersion. SSMF is one of the earliest single-mode fibers and, hence, has a relatively large installed base of infrastructure. As compared to the other single-mode fibers mentioned above, SSMF is the least expensive and was originally designed to reduce or eliminate modal dispersion, as compared to multimode optical fiber. The diameter of the core of SSMF, although still relatively small, is considered large when compared to some of the newer single-mode fibers such as NZDSF. For example, the core area for an SSMF may be 80 xcexcm2.
The average chromatic dispersion for SSMF may be relatively high, for example,   17  ⁢      xe2x80x83    ⁢      ps          nm      xc3x97      km      
for a WDM or DWDM optical signal. The average chromatic dispersion (xe2x80x9cDxe2x80x9d) is typically expressed as the chromatic dispersion at the optical wavelength of 1550 nm. When analyzing the chromatic dispersion of an SSMF as compared to optical signal wavelength, the chromatic dispersion curve generally increases, in a nonlinear manner, as the wavelength increases and graphically may generally be represented by a positive slope. An example of SSMF is SMF-28(trademark) optical fiber, which is manufactured by CORNING Incorporated in Corning, N.Y.
Chromatic dispersion may be compensated for in SSMF using an SSMF DCF designed especially for SSMF. For example, assuming an SSMF with an average chromatic dispersion of   17  ⁢      xe2x80x83    ⁢      ps          nm      xc3x97      km      
and a chromatic dispersion slope (xe2x80x9cSxe2x80x9d) of about       0.055    ⁢          xe2x80x83        ⁢          ps                        nm          2                xc3x97        km              ,
the ratio of D/S is around 309. As such, the SSMF DCF would, ideally, be provided at a ratio of D/S of around 309 so that the addition of the two ratios would be around one to compensate for both D and S of the SSMF. One example of an SSMF DCF used to achieve this would have a D of xe2x88x9290 and an S of       0.29    ⁢          xe2x80x83        ⁢          ps                        nm          2                xc3x97        km              ,
to achieve the desired ratio of D/S of 309. Unfortunately, it is not always easy to fabricate an SSMF DCF to counteract or compensate for both the chromatic dispersion D and the chromatic dispersion slope S of the SSMF. Further, SSMF DCF is often very long, expensive, and inconvenient or impossible to implement in some applications.
Around 1985, a new single-mode fiber, Dispersion Shifted Fiber (xe2x80x9cDSFxe2x80x9d), was introduced. DSF, which may also be referred to as standard DSF, was designed so that the zero chromatic dispersion wavelength of the fiber was at or near the minimum attenuation wavelength of the fiber. For example, if the attenuation of an optical signal for a DSF was at a minimum at the wavelength of 1500 nm, the characteristics of the DSF would be established so that the chromatic dispersion of the DSF was at or near zero at this same wavelength of 1500 nm. Because the dispersion characteristics are xe2x80x9cshiftedxe2x80x9d to correspond with the minimum attenuation point, these fibers are referred to as DSFs. When analyzing the chromatic dispersion of a DSF as compared to optical signal wavelength, the chromatic dispersion generally increases, in a nonlinear manner, as the wavelength increases and graphically may generally be represented by a positive slope. Unfortunately, DSFs are relatively expensive and are not particularly advantageous in handling WDM or DWDM optical signals. This is because WDM and DWDM optical signals contain optical signals of many different wavelengths, and, hence, many of these wavelengths are not at the wavelength value of minimum attenuation and near zero chromatic dispersion.
More recently, NZDSF was developed to address some of the limitations of SSMF and DSF, such as chromatic dispersion, and to increase the capability to carry WDM and DWDM optical signals. Some implementations of NZDSF have been optimized to provide a small amount of chromatic dispersion across a broad range of wavelengths. The average chromatic dispersion for NZDSF may be, for example,   4  ⁢      xe2x80x83    ⁢            ps              nm        xc3x97        km              .  
As compared to the other single-mode optical fibers discussed herein, the diameter of the core of NZDSF is relatively small. For example, the core area for an NZDSF may be around the range from 50 to 70 xcexcm2. The smaller cores of NZDSF may not be able to handle some of the higher power requirements of WDM and DWDM optical signals. Although reduced chromatic dispersion serves to reduce some of the negative effects associated with high chromatic dispersion, unfortunately, even a small amount of chromatic dispersion may serve as an impediment to increased data rates. As mentioned above, low chromatic dispersion may also result in cross-phase modulation.
When analyzing the chromatic dispersion of an NZDSF as compared to optical signal wavelength, the chromatic dispersion generally increases, in a nonlinear manner, as the wavelength increases and graphically may generally be represented by a positive slope. Examples of NZDSF are LEAF(trademark) fiber, which is manufactured by CORNING, Incorporated, and TRUE WAVE(trademark) fiber, which is manufactured by LUCENT TECHNOLOGIES. Even though the average chromatic dispersion of NZDSF is significantly less than that of SSMF, chromatic dispersion, especially in long spans between regenerators or compensators, must be compensated for in NZDSF. Chromatic dispersion is compensated for in NZDSF, similar to SSMF, using an NZDSF DCF designed especially for NZDSF. For example, assuming an NZDSF, such as the TRUE WAVE fiber by LUCENT TECHNOLOGIES, with a D of   4  ⁢      xe2x80x83    ⁢            ps              nm        xc3x97        km              .  
and a chromatic dispersion slope S of about       0.05    ⁢          xe2x80x83        ⁢          ps                        nm          2                xc3x97        km              ,
this results in a ratio of D/S of around 80. To fully compensate, the NZDSF DCF would, ideally, be provided at a ratio of D/S of around 80 so that the combination of the two ratios would be around zero to compensate for both D and S. NZDSF may have a ratio of D/S, for example, in the range of 0 to 100.
Unfortunately the relatively low D and the average to high S of NZDSF makes it very difficult, if not impossible with current technology, to design and engineer an NZDSF DCF with desirable optical characteristics that will fully counteract or compensate for the D and S effects of the NZDSF to produce a residual D and S at or around zero. This is because the relatively low D and the average to high S of NZDSF requires that the NZDSF DCF have either an extremely low D or an extremely large S to fully account for both the D and the S. One example of an NZDSF DCF is one recently made available by LUCENT TECHNOLOGIES for use with LUCENT""s TRUE WAVE NZDSF. This NZDSF DCF has a D of xe2x88x92100 and an S of xe2x88x920.65 to provide a D/S ratio of around 154. As can be seen from the calculations done above, this would not fully compensate for both the D and the S of LUCENT""s TRUE WAVE fiber, which has a D/S ratio of 80.
As mentioned above, while it is difficult to design an optical fiber to best accommodate one wavelength, it is significantly more challenging to make a fiber that works well for carrying many simultaneous wavelengths. It has proven exceedingly challenging and difficult to balance all of the various factors and limitations mentioned above to design an optical fiber that can be used to economically and accurately transmit high data rate and high bandwidth WDM or DWDM optical signals. Unfortunately, there are no available optical fibers that provide an adequate solution to this significant problem.
From the foregoing it may be appreciated that a need has arisen for a composite optical fiber transmission line and method that provides overall improved performance, such as an improved data rate and bandwidth, across a range of optical signal wavelengths, while still maintaining acceptable or minimal levels of chromatic dispersion. In accordance with the present invention, a composite optical fiber transmission line and method are provided that substantially eliminate one or more of the disadvantages and problems outlined above.
According to one aspect of the present invention, a composite optical fiber transmission line is provided for use in an optical fiber transmission system. The composite optical fiber transmission line includes a standard single-mode fiber, such as an SMF-28 fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The single-mode fiber receives an input optical signal at a first end and generates a single-mode optical signal at a second end. The dispersion shifted fiber receives the single-mode optical signal at a first end and generates an output optical signal at a second end. The dispersion compensating fiber receives the output optical signal and generates a chromatic dispersion compensated optical signal that, preferably, is at some desired residual chromatic dispersion level and residual chromatic dispersion slope. In one example, the residual chromatic dispersion of the composite optical fiber transmission line is zero or near zero, and the residual chromatic dispersion slope of the composite optical fiber transmission line is zero or near zero.
According to another aspect of the present invention, a method for making a composite optical fiber transmission line is provided that includes providing a standard single-mode fiber with a first end, a second end, and a length L1, providing a nonzero dispersion shifted fiber, such as a nonzero dispersion shifted fiber, with a first end, a second end, and a length L2, providing a dispersion compensating fiber with a first end and a length L3, and interfacing the various fibers. The fibers may be interfaced, in one embodiment, by interfacing the second end of the standard single-mode fiber to the first end of the nonzero dispersion shifted fiber, and interfacing the second end of the nonzero dispersion shifted fiber to the first end of the dispersion compensating fiber.
The present invention provides a profusion of technical advantages that include the capability to efficiently and effectively integrate significant portions of existing optical fiber installations and infrastructure of SSMF, such as SMF-28, with new optical fiber installations to provide a composite optical fiber transmission line that significantly increases the overall capacity or bandwidth of an optical fiber transmission system and telecommunications network. This can substantially increase overall network profitability and performance by allowing more information to be transmitted for only a moderate capital investment. The installation of optical fibers is expensive and time consuming and the present invention eliminates the costly proposition of abandoning or not using existing optical fiber installations when installing the next generation of optical fiber to increase overall bandwidth. The present invention provides substantial improvements in both the overall cost and time required to install optical fibers needed to increase bandwidth.
Another technical advantage of the present invention includes the capability to increase the bandwidth or data rate, using existing optical fiber installations with enhancements, by providing desirable optical characteristics that apply over a wide range of optical signal wavelengths. The provides a more robust optical fiber transmission system that is capable of successfully transmitting one or more optical signals having different optical frequencies or wavelengths, such as by using WDM or DWDM optical signals.
Yet another technical advantage of the present invention includes the capability to handle the additional power demand required by higher number of optical channels, such as WDM or DWDM optical signals. As the bandwidth or data rate increases, more power is required to transmit such optical signals. Optical fibers typically require a larger core to handle the increased power demand. The present invention may use existing installations of SSMF fiber to receive an optical signal when its power level or power density is at its highest, and then to use a smaller core NZDSF fiber to propagate the optical signal after the optical signal has attenuated or dissipated somewhat and does not need a larger core fiber. This also reduces wave mixing.
Still yet another technical advantage of the present invention includes a composite fiber with optical properties to reduce self phase modulation, cross phase modulation, decrease the BER and, hence, increase bandwidth, and decrease the need to use a long dispersion compensating fiber.
Still another technical advantage that the present invention provides includes the capability to operate an optical fiber telecommunications system to implement WDM or DWDM optical signals and to use an optical amplifier that can amplify all wavelengths of the optical signal without having to individually convert and amplify all optical wavelengths.
Another technical advantage of the present invention includes the capability to provide a residual chromatic dispersion that is at a desired value, such as zero or near zero, and to provide a residual chromatic dispersion slope that is at a desired value, such as zero or near zero.
The present invention provides all of the advantages of the latest and most advanced optical fiber, including the capability to handle DWDM and extremely fast data rates, while minimizing costs by leveraging the existing large infrastructure of installed SSMF, such as SMF-28.
Other technical advantages are readily apparent to one skilled in the art from the following figures, description, and claims.